Fatigue Delamination Behavior of PEEK Thermoplastic Composite Laminates

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Abstract
Edge delamination tension fatigue tests were conducted on AS4/PEEK composite lami nates to determine the fatigue delamination behavior of these graphite reinforced, semi crystalline thermoplastic matrix composites. The strain at onset of edge delamination, ∈ c, observed during fatigue tests of (35n/ — 35 n/0n/90n)s laminates, where n = 1,2, was plotted as a function of fatigue cycles, N. The delamination onset strain decreased dramatically with fatigue cycles and then began to level off to an endurance limit at 106 cycles. The delamination onset strains were used to calculate critical strain energy release rate, Gc, values as a function of fatigue cycles to provide a more generic representation of the fa tigue delamination behavior of the material, i.e., one that is independent of layup and ply thickness. Although the static interlaminar fracture toughness of the AS4/PEEK compos ite is much greater than the toughness of graphite epoxy composites, the delamination fa tigue Gc threshold, calculated from the cyclic strain endurance limit at 106 cycles, was only slightly greater than the delamination fatigue threshold for graphite epoxy composites. This comparison, however, was based only on the contribution of mechanical strain to the strain energy released. An asymmetric (0 2/906)T AS4/PEEK curved panel was heated in an oven to determine the stress free temperature of the composite, and quantify the contri bution of residual thermal stresses to the strain energy release rate at delamination onset. The contribution of residual thermal stresses to delamination in AS4/PEEK was substan tial due to the large temperature range between manufacture and room temperature. More work is needed to fully characterize the influence of residual thermal and moisture stresses, and the degree of crystallinity of the PEEK matrix, on the strain energy release rate for delamination growth, and the interlaminar fracture toughness of the composite.